Heretofore, optical elements for use in optical apparatuses, such as a camera, binoculars, a microscope, a semiconductor, and a liquid crystal exposure apparatus, have been provided with a black light-shielding film disposed as required to a region other than an optically effective area for reducing stray light. The stray light reaching a rim portion and the like of a lens is sufficiently absorbed by the light-shielding film, so that unnecessary light, such as flare and ghost, is reduced.
The light-shielding film is expected to have an effect of reducing stray light entering the light-shielding film from the inside of the optical element, and is demanded to suitably reduce reflected light referred to as inner reflection.
With a reduction in size and an increase in performance of optical apparatuses in recent years, materials with a higher refractive index have been used for optical elements for use in the optical system. In order to reduce internally reflected light, the refractive indices of the light-shielding film and a lens need to be approximated. Then, a method is known which introduces a component with a high refractive index into the light-shielding film to thereby suitably control the refractive index of the light-shielding film.
PTL 1 discloses a light-shielding film for optical element in which the refractive index is increased by the use of coal tar and internally reflected light is reduced by absorbing light by coal tar, carbon black, and a dye. Moreover, PTL 2 discloses a light-shielding film for optical element in which the refractive index is increased by the use of non-black inorganic fine particles and internally reflected light is reduced by absorbing light by a dye.
The light-shielding film is also required to have, in addition to the effect of reducing internally reflected light described above, durability under high temperature and high humidity conditions and resistance to a cleaning liquid, such as an organic solvent, to be used in a cleaning process of the optical element after the formation of the light-shielding film. In particular, a light-shielding film provided on the peripheral portion of a cemented lens in which glass lenses are cemented to each other is required to be heat treated at a temperature lower than the temperature of a single lens to be cured in order to prevent the separation of the interface of a bonding resin and the glass lenses due to a thermal expansion difference of the bonding resin and the glass lenses.